This is an example of how you can reduce memory usage for Tilesheet class in openfl with dumpBits() method. But you also listen for stage resize event and recreate tilesheet after context loss.

dumpBits example

import flash.Lib;
import flash.display.Sprite;
import flash.display.Bitmap;
import flash.display.BitmapData;
import openfl.Assets;
import openfl.display.Tilesheet;
import flash.display.Graphics;
import flash.events.Event;

class Particle
{
   var x:Float;
   var y:Float;
   var red:Float;
   var green:Float;
   var blue:Float;
   var alpha:Float;
   var angle:Float;
   var aspect:Float;
   var size:Float;

   var dx:Float;
   var dy:Float;
   var da:Float;
   var daspect:Float;


   public function new()
   {
      x = 320;
      y = 240;
      angle = 0;
      dx = Math.random()*2.0 - 1.0;
      dy = Math.random()*2.0 - 1.0;
      da = Math.random()*0.2 - 0.1;
      daspect = Math.random()*0.1;

      aspect = Math.random()*1 + 0.25;

      red = Math.random();
      green = Math.random();
      blue = Math.random();
      alpha = Math.random();

      size = Math.random()*1.9+0.1;
   }

   public function addSimple(data:Array<Float>)
   {
      data.push(x);
      data.push(y);
      data.push(0);
   }

   public function add(data:Array<Float>)
   {
      data.push(x);
      data.push(y);
      data.push(0);
      data.push(size);
      data.push(angle);
      data.push(red);
      data.push(green);
      data.push(blue);
      data.push(alpha);
   }

   public function addTrans(data:Array<Float>)
   {
      data.push(x);
      data.push(y);
      data.push(0);

      var t00 = size*Math.cos(angle);
      var t01 = size*Math.sin(angle);
      var t10 = - t01;
      var t11 = t00;

      var wobble = 1.0 + Math.cos(aspect)*0.75;
      data.push(t00 * wobble); // Squish in x-direction...
      data.push(t01 * wobble);

      data.push(t10);
      data.push(t11);

      data.push(red);
      data.push(green);
      data.push(blue);
      data.push(alpha);
   }


   public function addColoured(data:Array<Float>)
   {
      data.push(x);
      data.push(y);
      data.push(0);
      data.push(red);
      data.push(green);
      data.push(blue);
      data.push(alpha);
   }


   public function move()
   {
      var rad = 30 * size;

      x+=dx;
      if (x<rad)
      {
         x = rad;
         dx = -dx;
      }
      if (x>640-rad)
      {
         x = 640-rad;
         dx = -dx;
      }

      y+=dy;
      if (y<rad)
      {
         y = rad;
         dy = -dy;
      }
      if (y>480-rad)
      {
         y = 480-rad;
         dy = -dy;
      }

      angle += da;
      aspect += daspect;

   }
}

class Sample extends Sprite
{
   var tid:Int;
   var particles : Array<Particle>;
   var tilesheet : Tilesheet;
   var shape:flash.display.Shape;

   public function new()
   {
      super();

      Lib.current.stage.addChild(this);

      shape = new flash.display.Shape();
      var gfx = shape.graphics;
      gfx.beginFill(0xffffff);
      gfx.lineStyle(1,0x000000);
      gfx.drawCircle(32,32,30);
      gfx.endFill();
      gfx.moveTo(32,32);
      gfx.lineTo(62,32);
      var bmp = new BitmapData(512, 512, true, 0x00000000);
      bmp.draw(shape);
  
      tilesheet = new Tilesheet(bmp);
      tilesheet.addTileRect( new flash.geom.Rectangle(0,0,64,64), new flash.geom.Point(32,32) );
      tid = 0;
	  
	  bmp.dumpBits();

      particles = [];
      for(i in 0...100)
         particles.push( new Particle() );

      stage.addEventListener( Event.ENTER_FRAME, onEnter );
	  stage.addEventListener(Event.RESIZE, onResize);
   }
   
   private function onResize(e:Event):Void 
   {
	  var bmp = new BitmapData(2048, 2048, true, 0x00000000);
      bmp.draw(shape);
	  tilesheet = new Tilesheet(bmp);
      tilesheet.addTileRect( new flash.geom.Rectangle(0,0,64,64), new flash.geom.Point(32,32) );
      tid = 0;
	  
	  bmp.dumpBits();
   }

   public function onEnter(_)
   {
     var data = new Array<Float>();
      var flags = 0;
      particles[0].addSimple(data);
      graphics.drawTiles(tilesheet,data,true,flags);

      var data = new Array<Float>();

       var flags = Graphics.TILE_TRANS_2x2 |
                  Graphics.TILE_ALPHA | Graphics.TILE_RGB | Graphics.TILE_BLEND_ADD;
      for(p in particles)
      {
         p.move();
         p.addTrans(data);
      }
     
      graphics.clear();
      graphics.drawTiles(tilesheet,data,true,flags);
   }

   public static function main()
   {
      new Sample();
   }

}